1. Field of the Invention
The invention concerns a method of thermal treatment of metallic sheets comprising a thermal treatment step during which said sheets are in contact with each other over at least a part of their surfaces, and a step of preliminary treatment of said surfaces, the purpose of which is to prevent the sticking of said sheets at the level of said contacting surfaces.
2. Background of the Invention
When a thermal annealing treatment of steel sheets in coils or in tight sheets (e.g. stacked without separation wedges) is carried out, the different windings of the coil or the folded sheets often risk sticking or adhering to each other; this sticking or adhering is caused by the diffusion of the metal at the level of the contacts between the surfaces of the sheets. This risk of sticking appears notably in the case of alloy annealing.
The term "alloy annealing" is defined here as a thermal treatment for steel sheets coated with a metallic deposit intended to cause alloying between the iron of the substrate and the metal of the coating; the coating metal is, for example, zinc applied to the immersed material or electroplated nickel.
This type of annealing is generally carried out at a high temperature, which increases the risk of sticking: in the range 450-650.degree. C. for the alloying of a nickel coating, 250-400.degree. C. for the alloying of a zinc coating.
The document JP 01 298113 (1989) describes a treatment of metallic surface or polymer, prior to a treatment of cutting, welding or hardening, by laser application of the surface; this preliminary treatment consists in applying a coating agent comprising a powder with high conductivity, which is heat resistant, a tackifier heat-resistant agent chosen from acids, esters or stearate salts, organic polysilanes or silicone oils, polyethylenes or polybutenes and their derivatives, diluted and dispersed in an organic solvent with low boiling point, such as ethanol or CCl.sub.4.
Nothing in this document indicates that this preliminary treatment could limit the risks of sticking, since one of the intentions pursued here is, to the contrary, to make welding possible.
To limit the risk of sticking, it is known to carry out, prior to the thermal treatment, a treatment of the surface of the sheets, as described in the following documents:
JP 08 333689 A For the preparation for the thermal treatment of a nickel coated sheet, 0.1-2.5 mg/m.sup.2 of silica are first applied by immersing the sheet in a solution of sodium orthosilicate; PA1 JP 04 154973 For the preparation for the thermal treatment of steel wires, the wire is descaled by stripping, and then it is immersed in a solution based on silica and sodium hydroxide, and heated to a temperature of 70-90.degree. C. so as to form a deposit with a surface density of more than 3 g/m.sup.2 ; PA1 JP 55 82726 To avoid the sticking between windings during the basic annealing of steel types called "low carbon" steels, a solution of silicate or colloidal suspension of silica is first atomized on the sheets; the procedure is carried out in a solvent such as methanol. PA1 application is carried out so as to obtain, on said surfaces, a deposit of polysiloxane having a surface density less than or equal to 20 mg of silicon equivalent per m.sup.2 and per face, and PA1 the quantity of said silane introduced into the treatment solution is 1-15 vol %. PA1 in which the radicals X, which may be identical or different, are chosen from the group comprising the alkoxy, acyloxy, alkylcarbonyl or alkoxycarbonyl, alcohol radicals, or halogen; PA1 in which the radicals R, which may be identical or different, arc chosen from the group comprising the alkyl, alkenyl, alkynyl, aryl, arylalkyl, alkylaryl, arylalkenyl, alkenylaryl, arylalkynyl or alkylnylaryl carbon radicals, which can be interrupted by O, S or N atoms, and which can be substituted by one or more radicals chosen from the group comprising halogen amino, amide, aldehyde, keto, alkylcarbonyl, carboxy, alkoxycarbonyl, mercapto, cyano, hydroxy, alkoxy, alkoxycarbonyl radicals, sulfonic acid, phosphoric acid, acryloxy, methacryloxy, epoxy, vinyl, ureido, glylcidoxy, NH.sub.2,--CH.dbd.CH--, CH.sub.2 .dbd.CH--, CH.sub.2 .dbd.(CH.sub.3)(COO--), CH.sub.2 (--O--)CH--CH.sub.2 O--, HS--, Cl; PA1 in which m is equal to 0, 1, 2 or 3; PA1 although no degreasing of the surface is carried out before the application of the surface treatment solution; PA1 and even for deposits having a low surface density, less than 20 mg/m.sup.2 /face of silicon equivalent.
The drawback of these preliminary surface treatments intended to prevent sticking is that they must be carried out on clean surfaces, and even activated surfaces, which requires degreasing and even stripping of this surface before the application of the "antistick" deposit.